This invention relates, in general, to a process for forming a thin silver layer on metal leadframes used in manufacturing semiconductor products, and more particularly to a method for flashing silver on copper leadframes using no masks on the leadframes or outside electrical driving forces in the cleaning bath.
Electroplating metal leadframes with various other metals to obtain desired characteristics and properties is well known in the art. Copper leadframes are commonly electroplated with silver and/or tin to improve oxidation resistance and enhance thermal and electrical conductivities. However, current processes for electroplating metal leadframes are expensive and time consuming.
For example, semiconductor integrated circuit dual in-line packages (DIPs) and the like that employ a copper leadframe commonly have a thick layer of silver electroplated only on the flag. The thickness of this layer ranges between 75,000 and 90,000 angstroms and must be exact. In order for this to be accomplished, a mask or series of masks must be employed on a leadframe. The masked leadframe is then placed into an electrolytic bath containing silver in solution. The bath includes an anode and a cathode between which an electrical current flows. This serves as an outside electrical driving force to enhance electroplating speed. Following assembly of the semiconductor device on the leadframe and the plastic encapsulation of the device, the leads are then plated with tin to prevent oxidation and allow for improved thermal and electrical conductivities in the leads.
Arranging a mask or series of masks on a leadframe is an extremely time consuming and, therefore, expensive process. Masks must be exactly fitted and if they are not, silver whiskers result. The whiskers diffuse because of the material gradients and cause a non-homogenous surface on the leadframe which increases the difficulty in future processing steps as well as causing poor adhesion between the plastic encapsulation and the leadframe. The poor adhesion allows moisture to penetrate the semiconductor device.
The thick layer of silver which is utilized on the leadframe is extremely expensive. To obtain this thick layer, the leadframe must remain in the cleaning bath for a substantial period of time. In view of the above, a silver flashing process for metal leadframes which is inexpensive, has a short bath time and does not use excessive amounts of material will be extremely valuable.